Many optical transceivers operate based on coherent communications, in which the receiver has a prior knowledge of the type of the data being transmitted. For example, in heterodyne systems, the receiver may have information on a carrier frequency and a phase of the carrier frequency of a received modulated signal and can apply a corresponding local oscillator (LO) with proper frequency and phase to demodulate the received signal. Coherent communications provides orders of magnitude better receiver sensitivities over non-coherent detection (e.g., direct detection). For example, a coherent receiver may have a better sensitivity of about 20 to 40 photons/bit as compared to a typical receiver sensitivity of a non-coherent receiver of more than 1000 photons/bit. Additionally, LO gain enables use of noisier, but faster receivers (e.g., optical detectors) and enables much higher data rates (e.g., 40 to 100 Gbps) which is significantly higher than data rates (e.g., 10 Gbps) of non-coherent receivers.
Free space optical (FSO) communications can enable high-speed wireless communications over a sizable range (e.g., many kilometers). In terrestrial applications, atmospheric turbulence can significantly degrade performance. For example, the atmospheric turbulence can substantially reduce coherence of received (RX) light causing coherent systems to underperform direct detect systems.
An adaptive optics technique has been used to address the problem of recovering the spatial coherence of the received light disturbed by the atmospheric turbulences. This technique involves measuring the spatial phase variations of the incoming received light and using some sort of corrector (e.g., a deformable mirror) to flatten the phase of the received signal. This technique reduces the phase differences between the signal and the local oscillator and can improve the coherent SNR of the system. The adaptive optics systems, however, have to use expensive low-noise sensors and unreliable micro-mechanical mirrors as deformable mirrors. Further, the adaptive optics systems introduce additional optical losses into the system that adversely affect the optical throughput efficiency of the system.